Drawability of deoxidized steels by the addition of phosphorus and silicon

ABSTRACT

The high drawability of deoxidized steels with Mn contents below about 0.22 percent is enhanced by the addition of from about 0.015 to 0.05 percent P. The beneficial effect of such phosphorus additions on drawability is further enhanced by employing from about 0.02 to 0.06 percent Si.

United States Patent 1191 Ludwigson Dec. 16, 1975 DRAWABILITY OF DEOXIDIZED STEELS BY THE ADDITION OF PHOSPHORUS AND SILICON [75] Inventor: David C. Ludwigson, Hempfield Township, Westmoreland County, Pa.

[73] Assignee: United States Steel Corporation, Pittsburgh, Pa.

22 Filed: Sept. 30, 1974 21 Appl. No.2 510,844

[52] US. Cl. 148/36; 75/123 R [51] Int. C1. C22C 38/00 [58] Field of Search 75/123 R; 148/2, 3, 12,

[56] References Cited UNITED STATES PATENTS 3,215,567 11/1965 Yoshida 75/123 R X 1/1966 Canada 148/36 Primary Examiner-C. Lovell Attorney, Agent, or FirmArthur J. Greif [57] ABSTRACT The high drawability of deoxidized steels with Mn contents below about 0.22 percent is enhanced by the addition of from about 0.015 to 0.05 percent P. The beneficial effect of such phosphorus additions on drawability is further enhanced by employing from about 0.02 to 0.06 percent Si.

2 Claims, 2 Drawing Figures US. Patent Dec. 16, 1975 3,926,692

2 0.030 P. k 2.0- I 0.020 P. a E -0010 P. k v;

I r l l 1.4 l l A2 I l I I l l l .7 MANANESE CONTENT wr Ix 9 22 Q 0.020 P.

L6 l I l 2 WT SILICON DRAWABILITY F DEOXIDIZED STEELS BY THE ADDITION OF PHOSPHORUS AND SILICON This invention relates to an economical sheet steel product with exceptionally high deep drawability, as evidenced by r value, and is more particularly related to a specific range of steel compositions which will provide such deep drawability.

It is generally recognized that the performance of a sheet steel during forming operations known as deep drawing is closely associated with the ratio, r, of true width strain to true thickness strain when the steel is strained in tension in the length direction. Therefore, the suitability of a steel for deep drawing may be assessed by measuring r in the laboratory and the greater difficulty of full scale drawing trials can often be averted. It is normal to measure r in the plane of the sheet in three directions, parallel to the rolling direction (r diagonal to the rolling direction (r and perpendicular to the rolling direction (r From these three components, two summary characteristics are usually derived High values of F are associated with a high capability to undergo deep drawing with fracture; and values of Ar near zero are associated with a low tendency toward a detrimental directional nonuniformity in deep drawn items known as eating.

Isotropic steels have been produced with 7 and Ar values near 1.0 and 0.0, respectively. Such steels have limited deep drawability but excellent resistance to eating. Cold-rolled rimmed sheet steels generally exhibit Yand Ar of about 1.2 and +0.4, respectively. Such steels can be subjected to mild deep drawing operations, but develop detrimental earing. Drawing-quality special-killed (DQSK) steels are often characterized by Tand Ar values near 1.5 and +0.5, respectively. Although these steels can Withstand severe draws, they too tend to suffer from earing; and they are more costly to produce than rimmed steels. Recently sheet steels containing columbium or titanium to combine with interstitial elements have appeared. These steels have? values of 2.0 or more and thus can withstand very severe deep drawing. Earing tendency may be small in some instances, as indicated by Ar values near 0. l or high in other instances, as indicated by Ar values near +0.5. However, because of their columbium or titanium contents, these steels are very costly to produce. A significantly more economical method for improving deep drawability is through the addition of phosphorus to unkilled steels. Thus, U.S. Pat. Nos. 3,215,567 and 3,244,565 show that the addition of greater than about 0.03 percent phosphorus to an unkilled steel, followed by a decarburization of the sheet product, can significantly improve deep drawability. However, the r values of these steels only approach those of the above noted QSK steels.

It is therefore a principal object of this invention to provide a phosphorus containing steel composition which can yield sheet products with 7values of the order of 2.0 and greater.

It is a further object of this invention to provide a phosphorus containing steel composition which can provide superior drawability without the need for decarburization, i.e., in which the carbon content of the final product is greater than 0.010 percent.

These and other objects will be more readily understood from a reading of the following description when read in conjunction with the appended claims and drawings, in which:

FIG. 1 shows the interacting effects of Mn and P on r value, and

FIG. 2 shows the beneficial effect, on r value, of Si additions to a phosphorus containing steel.

The anisotropy that results in favorable 7 and Ar values is developed through metallurgical reactions in the steel that favor certain orientations of individual grains in the steel with respect to the steel surface and rolling direction. In particular, when a high fraction of grains are oriented with their 111 axes perpendicular to the face of the sheet, high 7values are observed. To determine the effect on such orientations, of those residual elements which are normally found in sheet steels, a series of heats were prepared with planned variations of the following elements; manganese, phosphorus, sulfur, silicon, copper, nickel, oxygen and nitrogen. Each heat was finish hot-rolled to a nominal thickness of 0.09 inch, at a temperature of about 1650F and the resultant band slow cooled from ll50F to simulate the thermal history of commercial coiled product. After descaling, each band was cold reduced to a nominal thickness of 0.030 inch, i.e., to an aim cold reduction of about 67 percent. The resultant cold-reduced strips were recrystallization-annealed in conventional manner and subsequently evaluated for both ?and Ar values. From the data on the experimental steel compositions, and with the use of regression analysis; the relationship of Fand Ar as a function of the levels of the above noted alloy elements was derived. It was found that within the ranges tested, that the elements sulfur copper, nickel, oxygen and nitrogen were all detrimental to the achievement of high T values. Stated another way, increasing the level of any of the foregoing five elements had an inverse effect on 7 value. In view thereof, it is desirable that the aforementioned steelmaking impurities be maintained at levels less than the following specified maximums: 0.04 percent S, preferably 0.03 percent max.; 0.05 percent Cu; 0.12 percent Ni; 0.015 percent N, preferably 0.01 max.; and 0.02 percent oxygen, preferably 0.015 percent max. However, in distinction to the teachings of the aforementioned U.S. Pat. Nos. (3,215,567 and 3,244,565) it was found that phosphorus can enhance the drawability of killed (deoxidized) steels, as well. However, with killed steels P will only be beneficial if the Mn level is maintained below about 0.22 wt. percent. The rather surprising interaction between Mn and P in a deoxidized steel will be more readily understood by reference to FIG. 1. It may be seen, in accord with the teaching of U.S. Pat. No. 3,215,567 (eg. compare therein steels 17 vs 18) that for a killed steel in which the level of Mn is, for example, 0.35 percent, that an increase in the level of phosphorus will have a significant, detrimental effect on drawability. With compositions containing from 0.22 to 0.28, the level of P has only a slight or negligible effect on ?value. More importantly, however, with Mn levels of about 0.22 percent or less, the T value materially increases with increasing phosphorus contents; this beneficial effect of phosphorus becoming even more pronounced with progressively decreasing manganese contents.

In view of this flip-flip effect of P in killed steels depending on Mn content, it may therefore be surmised why the above noted prior art teachings as to the bene- 3 ficial effects of P are limited to unkilled steels. The reduction of Mn to levels below about 0.2 percent is not only expensive, but may be detrimental in that there generally would be an insufficient amount to bind 4 measured and calculated strain-ratio values. Example 1, provided for comparison, is a control sample wherein p is outside the scope of this invention. Examples 2 and 3 are compositions within the scope of this up the S present in steel; thereby rendering the steel 5 invention.

Table I No. C Mn P S Si Cu Ni Mo N Al(tota1) O(ppm) subject to edgecracking (hot-shortness). Thus, the art Table II has only employed low Mn levels when it was abso- StraimRatio values lutely requlred, i.e., for 1mprov1ng the sag-resistance of No. r r r A r enamelmg steels. Equally important, untll recently, 1 L581 L795 24291 L866 0141 such low Mn steels were not capable of being recrystal- 2 1.891 2.227 2.573 2230 +0005 lized by a subcritical anneal. Thus, it was necessary that 3 L760 2306 2-844 2304 0004 such low Mn steels be recrystallized by normallizing; a

treatment which would destroy the attainment of high 7 values. It is, of course, now known (e.g. U.S. Pat. Nos. 3,668,016 and 3,709,744) that such low Mn steels can be subcritically annealed if they are deoxidized to a requisite extent and that such lower levels of Mn are in fact beneficial for the production of high 7 values. Therefore, the investigations of U.S. Pat. Nos. 3,215,567 and 3,244,565 were probably limited to the effect of P in steels with Mn levels above about 0.25 percent. However, in (deoxidized) steels with such higher Mn levels, it may be seen that increasing the P content will actually decrease the FValue attained, i.e., FIG. 1, and U.S. Pat. No. 3,215,567, noted above.

Additionally, it was found that higher levels of Si also enhanced drawability; with the beneficial effect increasing progressively as the P content was increased. This latter relationship is shown in FIG. 2. Referring to that figure, it may be seen that a P level of 0.01 percent, an increase in Si content provides some increase in F value. However, at a P level of 0.02 percent or greater, a similar increase in Si provides a rather dramatic increase in 7value.

Three steel heats of varying compositions were processed to sheet, in the manner outlined above. The specific compositions of the resultant sheet products are shown in Table I, while Table II presents both the C 0.010 0.1 Mn 0.22 max.

Si 0.02 0.06 oxygen 0.02 max.

balance Fe and incidental steelmaking impurities, said product having been produced from a steel melt which was killed in order to reduce the oxygen content thereof to said maximum percentage.

2. The sheet product of claim 1 wherein oxygen is below about 0.015 percent. 

1. A DEEP DRAWABLE KILLED-STEEL PRODUCT EXHIBITING AN R VALUE OF AT LEAST ABOUT 2.0, THE COMPOSITION OF WHICH CONSISTS ESSENTIALLY OF, IN WEIGHT PERCENT,
 2. The sheet product of claim 1 wherein oxygen is below about 0.015 percent. 